During the cooking of chemical cellulose pulp with continuous digesters it has been conventional to use a pre-treatment arrangement with a chip bin, steaming vessel and an impregnating chip chute, before the cooking process is established in the digester. Steaming has been carried out in one or several steps in the chip bin, prior to the subsequent formation of a slurry of the chips in an impregnation fluid or a transport fluid. The steaming has been considered to be absolutely necessary in order to be certain of expelling the air and water that is bound in the chips, such that the impregnation fluid can fully penetrate the chips, and such that air is not drawn into the system.
For example, U.S. Pat. No. 3,330,088 demonstrates the principle of such a system with a chip bin and a subsequent steaming vessel.
A great deal of development has taken place in order to optimise the steaming processing the chip bin, of which CA1154622, U.S. Pat. No. 6,199,299 and U.S. Pat. No. 6,284,095 only constitute examples of such development.
Attempts have been made to integrate the chip bin with the impregnation vessel in order to obtain in this manner a simpler system.
Already in U.S. Pat. No. 2,803,540, a system was revealed in which the chips from a chip bin were fed to a vessel in which a combined steaming and impregnation was achieved. In this vessel, the chips were steamed at the upper part of the vessel and impregnation fluid at the same temperature was added at various levels in the vessel.
These principles were applied in a process known as “Mumin cooking”, which is described in “Continuous Pulping Processes”, Technical Association of the Pulp and Paper Industry, 1970, Sven Rydholm, page 144. In this process, unsteamed chips were passed to a combined impregnation vessel, where steaming was obtained in the upper part, and to which impregnation fluid was added at a point in the upper part of the vessel during forced circulation. The impregnation fluid was in this case carried exclusively in the same direction of flow as the chips.
A system is shown in U.S. Pat. No. 5,635,025 in which the chips are fed without prior steaming to a vessel in the form of a combined chip bin, impregnation vessel and chip chute.
Steaming of the chips takes place here, the chips lying above the fluid level, and a simple addition of impregnation fluid takes place in the lower part of the vessel.
A further such system is revealed in U.S. Pat. No. 6,280,567, in which the chips are fed without prior steaming to an atmospheric impregnation vessel in which the chips are heated by the addition of warm black liquor that maintains a temperature around 130-140° C. The black liquor at high temperature is added just below the fluid level and is subjected to a reduction of pressure up through the bed of chips, after which foul-smelling released gases are removed from the top of the vessel. This creates large volumes of foul-smelling gases, which must be handled and destroyed in special systems. In this case, the impregnation fluid passes strictly in a concurrent flow direction, that is, impregnation fluid and chips move in a downwards direction.
An alternative system is revealed by SE,A,9802879-8, in which pressurised black liquor is added to the upper part of the steaming vessel, whereby the black liquor after being subjected to a pressure reduction releases steam for the steaming process. In this case, excess fluid, black liquor, can be drawn off from the lower part of the vessel.
The prior art has mostly exploited steaming as a major part of the heating of the chips, in which the steam that is used is either constituted by fresh steam or by steam flashed off from pressurised black liquor obtained from the cooking process. This involves a relatively large flow of steam, and its associated consumption of energy, and it requires a steaming system that can be regulated. The steaming has also involved the generation of large amounts of foul-smelling gases, and, at certain concentrations, a serious risk of explosion.
Problems arise when handling these volatile and readily condensed gases, which, for example, are constituted by turpentine and other hydrocarbons.
Special systems for handling these waste gases are required, and these must be dimensioned to cope with the volumes generated. Expensive systems with high capacity are required when these waste gases are created in large volumes.